The energy density in a high-speed and high-integration semiconductor is very high, and to efficiently exhaust the heat, a method in which the heat is quickly diffused and exhausted by using a heat radiation substrate made of aluminum or copper with excellent thermal conductance is suitable.
However, although the thermal expansion coefficient of ceramics to be used for the semiconductor or the circuit board of the semiconductor is 4 to 8×10−1 degrees centigrade, the thermal expansion coefficients of aluminum and copper are as high as 16 to 23×10−6/degrees centigrade, so that high heat stress occurs in the joining layer due to this difference in thermal expansion coefficient and it is not easy to join these materials together.
A first measure against this is to select a heat radiation substrate with a smaller thermal expansion coefficient, and conventionally, materials obtained by combining silicon carbide, tungsten, molybdenum, etc., having small thermal expansion coefficients with copper or aluminum metal with a high thermal conductivity, and to adjust the thermal expansion coefficient to 7 to 10×10−6/degrees centigrade are provided.
However, as a problem with these materials, the thermal conductivities of these materials are 20% or more lower than that of copper or aluminum alone such that the thermal conductivity of a material using copper is 200 to 300 W/m·K and the thermal conductivity of a material using aluminum is 150 to 200 W/m·K, and Young's modulus of the substrate is high, so that in the case of junction with silicon or aluminum nitride, etc., with a thermal expansion coefficient of about 4×10−6/degrees centigrade, the heat stress occurring in the joining layer increases, so that joining of a large area becomes difficult.
As a second measure, a resin or solder with a low Young's modulus is used for the joining layer to relax the heat stress to be caused by the difference in thermal expansion coefficient. However, this has a drawback in the thermal conductivities of the resin and solder being as low as 1 W/m·K and several tens of W/m·K, respectively, and small fracture strength, so that a thicker joining layer becomes necessary, and as a result, the thermal resistance of the joining layer increases.
In addition, disadvantages are also pointed out in that, in the case of resin, hygroscopic property and thermal resistance are low, and in the case of solder, the yield stress in a practical temperature range is low and thermal fatigue easily occurs.
As described above, in heat radiation systems widely employed in electronics devices currently, improvement in thermal conductivity in the joining layer which relaxes or reduces thermal stress caused by a difference in the thermal expansion coefficient has become an issue.
In the specification of this application, “thermal stress relaxing effect” means an effect in that, as known, a stress occurring in a joining interface of two materials with different thermal expansion coefficients when joining these is in proportion to the thermal expansion coefficients of the two materials and the elastic modulus of each material, so that the stress occurring in a material with a low elastic modulus becomes smaller, and even materials with greatly different thermal expansion coefficients can be joined, and can resist thermal fatigue caused by repetition of heating and cooling.
In view of these circumstances, previously, the inventor of the present invention suggested a metal impregnated carbon composite obtained by pressure-filling or impregnating the insides of pores of a carbon material such as graphite with a metal (for example, refer to Japanese Patent No. 3351778). In comparison with the above described material containing silicon carbide, tungsten, molybdenum, etc., in main proportions, this metal impregnated carbon composite has a high thermal conductivity, an equivalent thermal expansion coefficient, and a low Young's modulus, so that it shows an effect that relaxes thermal stress occurring in the joining layer of solder, etc., when silicon or ceramics, etc., is mounted, and it was found that this material improves the above described problem, however, it also includes disadvantages in that it is fragile and low in mechanical strength.
As a measure against this, the inventor of the present invention tried a method in which a metal impregnated carbon composite with a thickness of about 1 millimeter that was plated was joined onto a copper or aluminum substrate by soldering and a semiconductor element was joined by low-temperature solder thereon, however, the inventor found a problem in that the thermal expansion coefficient of the metal impregnated carbon composite was 4×10−6/degrees centigrade through 10×10−6/degrees centigrade, and on the other hand, the thermal expansion coefficient of copper was 16×10−6/degrees centigrade, and the thermal expansion coefficient of aluminum was 23×10−6/degrees centigrade, and these were greatly different, so that the solder-joined substrate warped to project toward the composite side, and trouble occurred in the post-process such as mounting of silicon. Under these circumstances, development of a heat radiation material with smaller warp and improved strength by applying a carbon material with a thermal stress relaxing effect has been demanded.
Patent document 1: Japanese Patent Publication No. 3351778